Current composite technologies developed for aircraft only allow for a low volume, autoclave-cure process that is both time-intensive and costly. The autoclave is often used to mold a prepreg, which is defined as a sheet of plurality of reinforcing fibers impregnated by a resin composition on one side or both sides of the plurality of the reinforcing fibers. Such a prepreg offers mechanical performance advantages over other forms of the fiber reinforced resins. A long cure cycle time results from both slow heating (ramp) rate of the autoclave and a prepreg material that needs to be isothermally cured over a long period of time to obtain a high degree of cure (DoC) to achieve the required thermal and mechanical performance. High ramp rates in the autoclave sometimes are not desired due to exotherm (amount of heat generated during cure but ineffectively controlled), especially for a composite article with thick dimensions.
The resin composition may contain a thermosetting resin and a curing agent that chemically interact. However, the interactions could occur at ambient conditions leading to undesired effects on the handling and processing abilities of the prepreg such as tack, drape, storage time, and out time (at certain processing conditions). Several approaches have been patented to improve handling and processing abilities of a prepreg employing a surface layer with substantially lower reactivity (than the resin composition that is used to impregnate the reinforcing fibers) applied to one, or both side of the prepreg. For example, US20140057515A1 (Suzuki et al., Toho) disclosed a composition of a prepreg comprising reinforcing fibers impregnated by a primary epoxy resin composition and a surface layer comprising a secondary epoxy resin composition, such that only either of the primary resin composition or the secondary resin comprises a curing agent. EP1072634A1 (Steele, Hexcel) suggested to have either one of their resin compositions of a prepreg comprise a major amount of a curing agent.
In order to reduce the cure cycle time, traditional approaches are to introduce an accelerator, a catalyst or a highly reactive curing agent into the resin composition to speed up its reaction kinetics (see, for examples, US20140309336A1, Hughes et al., Toray Industries and WO2010099029A1, Bongiovanni, Cytec). However, the accelerator might even accelerate the reactions at ambient or other suitable processing conditions, complicating not only the resulting prepreg's handling and processing abilities when not cured but also its thermal and mechanical properties after being cured.